This section provides background information related to the present disclosure which is not necessarily prior art.
In general, since the liquid crystal of a liquid crystal displays (LCD) is itself not a light-emitting element (unlike other types of flat panel displays), an additional light-emitting device is necessary to improve the brightness of the display screen.
For such a light-emitting device, different types of methods can be used, such as a frontlight technique or a backlight technique. The backlight technique is an indirect lighting method in which the light is emitted from a light source of a backlight unit mounted on the rear side of a display device, is converted into a primary planar light through an light guide plate LGP) or a diffusion plate, and is reflected by a reflection plate such as a metal deposit plate or an opaque white plate so as to emit the light onto a front surface of the display panel, thereby improving the brightness of the display screen. In such a backlight technique, the best image quality should be implemented with minimum energy consumption.
Conversion into a planar light source is important in the backlight technique. A general planar light source uses a variety of diffusion plates which irregularly diffuse light flux to secure brightness uniformity at the light flux emitting-surface. Because of diffusion, light flux emitted from the emitting-surface does not have a specific direction but radiates over a large area. Accordingly, various optical sheets are mounted on the backlight unit to focus the emission direction of light flux onto a smaller area and to control the light emission in a desired direction. For example, as a method to improve the viewing angle dependency of LCDs (which has been an obstacle for the expansion of application of LCDs), techniques have been presented to mount various light diffusion sheets or microlens array sheets onto diffusion plates or LGPs.
However, though such a method described above is effective in controlling a viewing angle in predetermined uniform directions, it is not effective in controlling the viewing angle in irregular directions. In other words, a construction of backlight sheet of LCDs is needed which can enhance the efficiency of the usage of light rays within a viewing angle of a certain direction.